Tunnel-emission triodes



March 22, 1966 H. JACOBS TUNNEL-EMISSION TRIODES Filed June 4., 1962 INVENTOR,

HAROLD JACOBS ATTORNEY.

United States Patent 3,242,396 TUNNEL-EMISSION TRIODES Harold Jacobs, West Long Branch, N.J., assignor to the United States of America as represented by the Secretary 0f the Army Filed June 4, 1962, Ser. No. 200,003 2 Claims. (Cl. 317235) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to improved solid-state devices and more particularly to such devices operating on the principle of tunnel emission.

Tunnel emission is the phenomenon occurring at a metal-insulator interface when a high electric field is present within the insulator. The phenomenon is most easily observed with reference to a diode structure consisting of a first and second metal plate separated by a thin insulating layer. The second plate is thinner than the first plate with a potential applied between the plates. A tunnel triode structure is constructed by adding a second insulating layer to the thin metal plate and then a third metal plate. By biasing the third metal plate positively, electrons emitted from the surface of the thin metal plate are collected at the third metal plate. The tunnel triode device thus formed is similar to a transistor, and the same terminology is applied to the metal plates; the first metal plate being the emitter, the thin metal region being the base, and the third metal plate being the collector. The general principles of tunnel triodes are described in a paper entitled, The Tunnel-Emission Amplifier, appearing in volume 48, No. 3, of the Proceedings of the IRE, March 1960, pages 359-661.

While these prior tunnel triodes indicated that they are inherently capable of extremely high frequency performance, it has been found difficult to fabricate satisfactory tunnel triode devices because the fraction of emitter current which actually reaches the collector, the device current gain, is quite low.

One general object of this invention is to improve the performance characteristics of tunnel triode devices.

More specifically, objects of this invention are to increase the current gain, increase the output resistance and lower the input resistance in such-devices,

An illustrative embodiment of the invention comprises a tunnel triode comprising a metallic emitter, a metallic base and a metallic collector, each of said metallic elements being separated by a layer of insulating material, the layer of insulating material between said metallic base and said metallic collector being magnesium oxide.

As will become apparent from the following detailed description, a limiting factor for effectively providing the tunnel triode with a high output resistance and a high output gain rests in the insulating layer between the base and the collector, which, in prior devices, was fabricated of silicon monoxide and was of the order of several hundred Angstroms in thickness. Basically, this insulating layer must be thick enough to prevent electrical leakage from the metal base to the metal collector. However, when it is thick enough to give adequate resistance it also tends to contribute a large number of trapping states into the surface of the insulating layer. When these traps become charged, the potential energy is increased and the emitter electrons cannot penetrate through to the collector, thus providing a low power.

In accordance with one feature of this invention, an avalanching type of insulator is utilized, so that the insulating material separating the base and the collector can still be relatively thick in order to provide high output resistance, and yet eliminate the problem of charging electrons which prevent the passage of emitter electrons through the material to the collector.

For a more detailed description of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, which is a cross-sectional schematic view of a tunnel triode according to the invention.

Referring to the drawing, there is shown a tunnel triode 10. For purposes of illustration, a relatively thick aluminum emitter layer 12 is evaporated, on a glass substrate 14, in the form of a stripe approximately 5 millimeters wide. Layer 12 is anodized in a nonsolvent electrolyte. This method produces on emitter layer 12, an aluminum oxide film 16 of the desired thickness in the range of 50 to Angstroms. To avoid field concentrations at the edges of film 16, silicon monoxide 18 is evaporated over all but a 1 millimeter stripe 20 in the center of the aluminum oxide film 16. A thin aluminum base layer 22 of about 100 Angstroms in thickness is evaporated through a mask onto film 16. The mask permits an aluminum band approximately 1 millimeter Wide to contact stripe 20 of the aluminum oxide film 16, and also allows the aluminum base layer 22 to extend to one side of the device 10 so that contact can be made to the base. A film of magnesium oxide 24, substantially 1 micron in thickness, is evaporated over the central part of base layer 22. Finally, a thick aluminum collector layer 26 is evaporated over the magnesium oxide film 24 in registry with stripe 20, and extending to a side of device 10 for contact purposes.

In the embodiment described above, the emitter 12 is made of aluminum simply because of the ease with which a thin film 16 of aluminum oxide, of known thickness,

could be formed on the surface of emitter 12 by anodiz-" ing. However, copper and nickel are other examples of suitable metals which can be used for the emitter 12, base 22, and collector 26. Also, film 16, intermediate emitter 12 and base 22, may be fabricated of magnesium oxide, substantially 1 micron in thickness, to even further improve the performance characteristics of device 10.

A tunnel triode 10 is thus described in which three metallic electrodes 12, 22 and 26 are in juxtaposition. These respective electrodes 12, 22 and 26 have the properties of emitter, base and collector in that order, and have a film of magnesium oxide, which is an avalanching type of insulator, intermediate the adjacent metallic electrodes. The magnesium oxide film 24 particularly provides an increased current gain in that the collector current is multiplied by avalanching. At the same time that the ratio of collector current to emitter current (alpha) is increased, the output resistance of device 10 will become considerably higher than that provided by conventional tunnel barriers because the mechanism of the avalanching process requires a higher change in voltage for a given change in current.

Considerable work has been done along these lines of avalanching insulators by the applicant, Harold Jacobs, and co-workers. In Solid State Physics, Frederick Seitz and David Turnbull, volume 6, pp. 306311, Academic Press, Inc., 1958, New York, is described the theory involved and its application with respect to field-dependent secondary emission of magnesium oxide films. According to the theory, it is believed that when a high voltage is applied across the magnesium oxide film 24, the electrons, that have tunneled into the magnesium oxide, will avalanche by secondary emission, or internal breakdown, in such a way as to create additional electrons on the passage of the carriers from the metallic base 22 to the metallic collector 26. The avalanching will remove all of the electrons which have been trapped in the magnesium oxide film, the replacement electrons being supplied by field emission from the metal in base 22.

Although the example described utilized magnesium oxide, it is to be understood that the principles of the invention are applicable to other avalanching materials such as the oxides and fluorides of magnesium and barium having a thickness of substantially 1 micron. These insulating materials when used in the regions intermediate the adjacent emitter 12, the base 22 and collector 26 make possible higher current gain and higher output resistance in tunnel triodes.

While there has been described what is at present considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein Without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A tunnel triode comprising three metallic electrodes in juxtaposition, the respective metallic electrodes having the properties of emitter, base and collector in that order, a film of magnesium oxide having avalanching characteristics intermediate adjacent metallic electrodes, and said References Cited by the Examiner UNITED STATES PATENTS 9/1962 Mead 317-234 2/1964 Davis 30788i5 OTHER REFERENCES Solid State Physics, Frederick Seitz and David Turnbull, vol. 6, pp. 306-311, Academic Press, Inc., 1958-, New York.

JOHN W. HUCKERT, Primary Examiner.

GEORGE WESTBY, Examiner.

R. F. POLISSACK, Assistant Examiner. 

1. A TUNNEL TRIODE COMPRISING THREE METALLIC ELECTRODES IN JUXTAPOSITION, THE RESPECTIVE METALLIC ELECTRODES HAVING THE PROPERTIES OF EMITTER, BASE AND COLLECTOR IN THAT ORDER, A FILM OF MAGNESIUM OXIDE HAVING AVALANCHING CHARACTER- 